Ok so I downloaded some free 3d arial scans of my hometown in dwg-format from the web. I isolated my school and now wanted to make it a 3d-printable stl-file.

Problem is that the files I downloaded are generated hollow. How do I easily make them solid? I can export them fine as stl, but they are hollow.

Also the buildings are at various heights and angled at the bottom because the ground is not the same height. How do I easily generate a terrain between the buildings groundlevel so they are not floating and can be printed as one piece?

Any help and input will be greatly appriciated. I have tried youtube, google and looking through extensions but to no real help.
I have uploaded the file here.

Thank you so much for taking the time to reply. I guess your suggestions would work well - but I am thinking there must be an easier way to automate more of the process. I suspect similar 3dmodels have been downloaded by many before me of other cities and I cant believe someone hasnt found a way to generate a printable stl from them without all that timeconsuming editing. I really appriciate your elaborate advice but I admit I was hoping for something… easier. I hope someone still has a suggestion. I dont mean to sound ungreatful but I would like to do this to -alot- of buildings… ( :

Thank you for your input. It sounds do able. I will try to figure out excatly how that is done. I have only worked with tinkercad.com before so I have a lot of lingo and functions to learn from the way sketchup works. You wouldnt happen to know of a video that shows the process you speak of?

If you select a group or component and use the solid inspector 2 tool, then it will tell you how to make it a solid. Sometimes it can even do certain things for you, like deleting stray geometry or internal faces. It makes the process a lot easier.